A thermoplastic resin film is widely used in magnetic tapes, capacitors, for photography and electric insulation, and as a packaging material or the like by utilizing its physically and chemically excellent properties.
Conventionally, there has been known a method in which fine grains of various types of inorganic compound such as silicon dioxide, titanium dioxide, and calcium carbonate or the like are contained in the film in order to improve the surface slipperiness as well as the wear resistance thereof by giving appropriate irregularities to the surface of the film, and a method using particles of silica and calcium carbonate or the like is disclosed in, for instance, Japanese Patent Laid-open Publication No.98729/1986, and a method using a combination of two kinds of silica particles each having a different diameter is also disclosed in Japanese Patent Laid-Open Publication No.284534/1989.
Although the slipperiness (anti-blocking property) and the wear resistance can be improved slightly by these methods described above, the inorganic oxide particles described above have not sufficient adhesiveness and compatibility with the thermoplastic resin such as polyester or the like, so that, when a film is fabricated in a elongation process or other processes similar to that, the separation occurs easily on the interface between the inorganic fine grains and polyester, which causes voids to be generated therein. In the film with the voids, hazing value of the film becomes remarkable, and when it is used as a magnetic tape or the like, the inorganic fine grains easily drop out or are easily cut away due to contact with other materials or with the film itself. For this reason, some dropouts or flaws on the surface thereof are generated as the slipperiness or the wear resistance becomes lower.
On the other hand, in recent years it has been required to make films further thinner in association with the demands for high-density of magnetic recording materials and for more compact size of devices using the same. For that purpose it is indispensable to improve a elongation factor and a elongation speed of the films. However, when a film is made thinner, stress is concentrated onto the interface between the inorganic fine grains and polyester described above. As a result, voids are generated on the interface and furthermore inorganic particles easily drop cut.
As a means to solve the problems described above, there are disclosed a method of processing the surfaces of inorganic particles with a silane compound in Japanese Patent Laid-Open Publication No. 304038/1988, and also a method using particles modified with an isocyanate compound and a water-soluble polyester compound on the surface of the inorganic particles, so called particles with the surfaces reformed, in Japanese Patent Laid-Open Publication No. 309551/1992.
However, even if any of these method have been employed, the adhesion and the compatibility between inorganic particles and polyester are not sufficient, so that the generation of voids thereon is not effectively suppressed, nor are the slipperiness and the wear resistance improved effectively.